Add semigroup instance for Tuple2 (#1321)

* Add monad defer laws ported from cats

* Remove stack safety laws for now as I don't think they apply

* Add arbitrary sleep to catch any async coroutine executions

* change method name: bind => flatMap

* Add delay constant func and make async and invoke start lazy by default

* Remove delay override in async instances

Monad defer already provides a default for it. And if async override differs in any way this would produce unexpected results

* Add test

* Add stack safety laws back

* Missing shouldBe true check

* Skip stack safety laws for reactor and rx2 integrations

* Add some documentation for rx2 and reactor not being stack safe

* Slight changes, remove attempt

* Fix links and names

* Add semigroup for Tuple2

* Fix instances

* Formatting

Author: pakoito

modules/core/arrow-core-extensions/src/main/kotlin/arrow/core/extensions/tuple.kt

@@ -1,4 +1,5 @@
 @file:Suppress("UnusedImports")
+
 package arrow.core.extensions
 
 import arrow.Kind
@@ -96,21 +97,33 @@ interface Tuple2Traverse<F> : Traverse<Tuple2PartialOf<F>>, Tuple2Foldable<F> {
 }
 
 @extension
-interface Tuple2Monoid<A, B> : Monoid<Tuple2<A, B>> {
-
-  fun MA(): Monoid<A>
+interface Tuple2Semigroup<A, B> : Semigroup<Tuple2<A, B>> {
 
-  fun MB(): Monoid<B>
+  fun SA(): Semigroup<A>
 
-  override fun empty(): Tuple2<A, B> = Tuple2(MA().empty(), MB().empty())
+  fun SB(): Semigroup<B>
 
   override fun Tuple2<A, B>.combine(b: Tuple2<A, B>): Tuple2<A, B> {
     val (xa, xb) = this
     val (ya, yb) = b
-    return Tuple2(MA().run { xa.combine(ya) }, MB().run { xb.combine(yb) })
+    return Tuple2(SA().run { xa.combine(ya) }, SB().run { xb.combine(yb) })
   }
 }
 
+@extension
+interface Tuple2Monoid<A, B> : Tuple2Semigroup<A, B>, Monoid<Tuple2<A, B>> {
+
+  fun MA(): Monoid<A>
+
+  fun MB(): Monoid<B>
+
+  override fun SA(): Semigroup<A> = MA()
+
+  override fun SB(): Semigroup<B> = MB()
+
+  override fun empty(): Tuple2<A, B> = Tuple2(MA().empty(), MB().empty())
+}
+
 @extension
 interface Tuple2Eq<A, B> : Eq<Tuple2<A, B>> {